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Burl on a sequoia tree. Photo: <a href="">Troy Hicks</a>, CC <a href="">some rights reserved</a>
Burl on a sequoia tree. Photo: Troy Hicks, CC some rights reserved

Natural Selections: Burl wood

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Burl wood, the knobs of complex grain that some trees form, is prized by woodworkers for its beauty and utility. What causes wood grain to deviate from the straight and narrow in this way is something of a mystery. Martha Foley and Curt Stager try to untangle the knot.

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There are few leads as to what causes a burl to occur. “In most of the cases it looks like probably the best estimate of what it is, is some sort of a benign tumor,” says Stager. There is no place or specific tree type where they are found.

Burls are random, densely packed grain, frequently with circular patterns which often contain buds or dormant growth tissues, explained Stager. In some cases with certain kinds of trees, those buds can be used to sprout and replace the tree in case the tree was burned or broken.

It is common that Redwoods have swelling on the base of the trunk that can actually grow over its roots. This swelling looks like a burl, but is actually called a bud collar or lignotuber which contain buds that can be released to grow if the top of the tree is damaged.

Burls are very attractive looking and are often used to create furniture, gunstocks and other wooden objects such as pipes for smoking. However, burls are weak compared to regular wood, so they are not typically used for anything structural.

People speculate that many Elm logs imported to the U.S. from France in the early 1900s had Dutch Elm disease. The trees were brought in to use their burls for making veneer furniture, and it is thought these trees are what spread the disease to this country.

Some tree diseases such as fungal diseases can be mistaken for burls. For example, Black Cherry trees may have a “black knot fungus thing,” says Stager, which forms a black lump on a twig that will get larger and larger. It may look like a burl from a distance, but up close you can tell it is not made of wood. After several years it can infect the Cherry tree and cause swelling, but that’s not a burl either.

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